Methods for segmenting x-ray images are known in principle. They can be used for diagnostic purposes to identify image regions in an x-ray image in which only a specific substance, for example bone or contrast medium, is mapped. The segmentation is usually performed on the basis of a modified image that is obtained by respectively determining two attenuation values relating to two differently set energies of an x-radiation in relation to each pixel, and subsequently subtracting them from one another in a weighted fashion.
Because of the fact that the absorption coefficients depend differently on energy from substance to substance, given an appropriate selection of the weighting coefficients this procedure assigns high intensity values in the modified image only to those pixels that are to be assigned to a specific substance. The pixels potentially belonging to a specific substance are segmented by means of a pixel-related threshold value comparison of the individual intensity values with a threshold value.
In many situations, however, such a threshold value comparison leads to an erroneous result, since the attenuation values can be falsified by measurement noise such that the threshold value criterion for segmenting a pixel is not fulfilled despite the presence of the substance.
U.S. Pat. No. 6,754,298 B2 discloses a reconstruction method in which there is calculated from a plurality of measurements acquired in relation to different energy spectra of the x-radiation an image in which the components of the object are illustrated in conjunction with a reduced noise component. The reconstruction method is based on an iterative reconstruction algorithm that requires long computing times and also takes account of the diversity of the energy spectra used to acquire the measurements when calculating the result image.